Date:   Fri, 22 Nov 2019 15:10:44 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        john fastabend <john.fastabend@...il.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 5/8] bpf: add poke dependency tracking for
 prog array maps

On Fri, Nov 22, 2019 at 3:06 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 11/22/19 11:55 PM, Andrii Nakryiko wrote:
> > On Fri, Nov 22, 2019 at 12:08 PM Daniel Borkmann <daniel@...earbox.net> wrote:
> >>
> >> This work adds program tracking to prog array maps. This is needed such
> >> that upon prog array updates/deletions we can fix up all programs which
> >> make use of this tail call map. We add ops->map_poke_{un,}track()
> >> helpers to maps to maintain the list of programs and ops->map_poke_run()
> >> for triggering the actual update.
> >>
> >> bpf_array_aux is extended to contain the list head and poke_mutex in
> >> order to serialize program patching during updates/deletions.
> >> bpf_free_used_maps() will untrack the program shortly before dropping
> >> the reference to the map. For clearing out the prog array once all urefs
> >> are dropped we need to use schedule_work() to have a sleepable context.
> >>
> >> The prog_array_map_poke_run() is triggered during updates/deletions and
> >> walks the maintained prog list. It checks in their poke_tabs whether the
> >> map and key is matching and runs the actual bpf_arch_text_poke() for
> >> patching in the nop or new jmp location. Depending on the type of update,
> >> we use one of BPF_MOD_{NOP_TO_JUMP,JUMP_TO_NOP,JUMP_TO_JUMP}.
> >>
> >> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> >> ---
> >>   include/linux/bpf.h   |  12 +++
> >>   kernel/bpf/arraymap.c | 183 +++++++++++++++++++++++++++++++++++++++++-
> >>   kernel/bpf/core.c     |   9 ++-
> >>   kernel/bpf/syscall.c  |  20 +++--
> >>   4 files changed, 212 insertions(+), 12 deletions(-)
> >>
> >
> > [...]
> >
> >> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> >> index 5a9873e58a01..bb002f15b32a 100644
> >> --- a/kernel/bpf/syscall.c
> >> +++ b/kernel/bpf/syscall.c
> >> @@ -26,12 +26,13 @@
> >>   #include <linux/audit.h>
> >>   #include <uapi/linux/btf.h>
> >>
> >> -#define IS_FD_ARRAY(map) ((map)->map_type == BPF_MAP_TYPE_PROG_ARRAY || \
> >> -                          (map)->map_type == BPF_MAP_TYPE_PERF_EVENT_ARRAY || \
> >> -                          (map)->map_type == BPF_MAP_TYPE_CGROUP_ARRAY || \
> >> -                          (map)->map_type == BPF_MAP_TYPE_ARRAY_OF_MAPS)
> >> +#define IS_FD_ARRAY(map) ((map)->map_type == BPF_MAP_TYPE_PERF_EVENT_ARRAY || \
> >> +                         (map)->map_type == BPF_MAP_TYPE_CGROUP_ARRAY || \
> >> +                         (map)->map_type == BPF_MAP_TYPE_ARRAY_OF_MAPS)
> >> +#define IS_FD_PROG_ARRAY(map) ((map)->map_type == BPF_MAP_TYPE_PROG_ARRAY)
> >>   #define IS_FD_HASH(map) ((map)->map_type == BPF_MAP_TYPE_HASH_OF_MAPS)
> >> -#define IS_FD_MAP(map) (IS_FD_ARRAY(map) || IS_FD_HASH(map))
> >> +#define IS_FD_MAP(map) (IS_FD_ARRAY(map) || IS_FD_PROG_ARRAY(map) || \
> >> +                       IS_FD_HASH(map))
> >>
> >>   #define BPF_OBJ_FLAG_MASK   (BPF_F_RDONLY | BPF_F_WRONLY)
> >>
> >> @@ -878,7 +879,7 @@ static int map_lookup_elem(union bpf_attr *attr)
> >>                  err = bpf_percpu_cgroup_storage_copy(map, key, value);
> >>          } else if (map->map_type == BPF_MAP_TYPE_STACK_TRACE) {
> >>                  err = bpf_stackmap_copy(map, key, value);
> >> -       } else if (IS_FD_ARRAY(map)) {
> >> +       } else if (IS_FD_ARRAY(map) || IS_FD_PROG_ARRAY(map)) {
> >
> > Why BPF_MAP_TYPE_PROG_ARRAY couldn't still stay as "IS_FD_ARRAY"?
> > Seems like it's still handled the same here and is technically an
> > array containing FDs, no? You can still have more precise
>
> For the update and delete method we need to hold the mutex in prog array
> which is why we cannot be under RCU there. For the lookup itself it can
> stay as-is since it's not a modification hence the or above.
>
> > IS_FD_PROG_ARRAY, for cases like in map_update_elem(), where you need
> > to special-handle just that map type.
> >
> >>                  err = bpf_fd_array_map_lookup_elem(map, key, value);
> >>          } else if (IS_FD_HASH(map)) {
> >>                  err = bpf_fd_htab_map_lookup_elem(map, key, value);
> >> @@ -1005,6 +1006,10 @@ static int map_update_elem(union bpf_attr *attr)
> >>                     map->map_type == BPF_MAP_TYPE_SOCKMAP) {
> >>                  err = map->ops->map_update_elem(map, key, value, attr->flags);
> >>                  goto out;
> >> +       } else if (IS_FD_PROG_ARRAY(map)) {
> >> +               err = bpf_fd_array_map_update_elem(map, f.file, key, value,
> >> +                                                  attr->flags);
> >> +               goto out;
> >>          }
> >>
> >>          /* must increment bpf_prog_active to avoid kprobe+bpf triggering from
> >> @@ -1087,6 +1092,9 @@ static int map_delete_elem(union bpf_attr *attr)
> >>          if (bpf_map_is_dev_bound(map)) {
> >>                  err = bpf_map_offload_delete_elem(map, key);
> >>                  goto out;
> >> +       } else if (IS_FD_PROG_ARRAY(map)) {
> >> +               err = map->ops->map_delete_elem(map, key);
> >
> > map->ops->map_delete_elem would be called below anyways, except under
> > rcu_read_lock() with preempt_disable() (maybe_wait_bpf_programs() is
> > no-op for prog_array). So if there is specific reason we want to avoid
> > preempt_disable and rcu_read_lock(), would be nice to have a comment
> > explaining that.
>
> See answer above. I didn't add an explicit comment there since it's not done
> for all the others either, but seems obvious when looking at the map implementation
> and prog_array_map_poke_run() / bpf_arch_text_poke().

Ok, fair enough. I was a bit lazy in trying to figure this out :)

Acked-by: Andrii Nakryiko <andriin@...com>

>
> Thanks,
> Daniel

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